A request for confidentiality in communications has been an eternal theme from ancient times through the future, and in the recent network society, the request has been secured by development of cryptology. Security of a public key cryptosystem etc. which are presently prevalent is based on the fact that it takes unrealistically long time to decipher the ciphertext, but since computer technologies continue advancing steadily, the security of the public key cryptosystem etc. is not always promised into the future. On the other hand, quantum cryptography which is currently actively studied has security promised by physical laws and even if the technology advances, its security will not be broken and realization of such security has been desired.
The quantum cryptography with the highest possibility to realization at present is a quantum key distribution system using faint LD light (Non-Patent Document 1: N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Reviews of Modern Physics 74, 145-195 (2002)). This system uses the quantum mechanical law for sharing a common key required between a sender and a receiver, and carries out general cipher communications after the common key is shared. In a process to share the common key, a dedicated optical line is used, and one signal is composed with faint light in which the number of photons is one or less, whereby a random number signal is transmitted. Because the number of photons in one signal is one or less, even if eavesdropping occurs, the legitimate receiver can discover the fact of the eavesdropping. Therefore, the legitimate receiver can establish the common key by using only the random number data that is deemed to be received safely without eavesdropping. The security of the system is proven even cryptographically, but the system requires a dedicated line and, in addition, because the number of photons is one or less for one signal, the signal is extremely susceptible to transmission loss and, for example, when 100 km transmission is conducted, a key generation rate becomes about several bps. Due to these defects, it is assumed that introduction of the quantum key distribution system using the faint LD light would be restricted to limited applications.
Against this, Yuen et al. have proposed a quantum mechanical system which do not only distribute keys but also transmits signals themselves by use of mesoscopic number of photons (“mesoscopic” means an intermediate between “macroscopic” and “microscopic”) (Non-Patent Document 2: G. A. Barbosa, E. Corndorf, P. Kumar, and H. Yuen, Physical Review Letters 90, No. 22, 227901 (2003)). Two quadrature phase components of light (or a pair of intensity and phase) are never fixed simultaneously in the accuracy that is less than quantum mechanical fluctuation. In the event that the transmission basis is finely varied in an optical transmission and detection system using phase-shift keying and that adjacent transmission bases are included in a quantum fluctuation range, eavesdroppers who do not know the transmission basis are unable to take out meaningful information from the eavesdropped signals. In this system, the basis is uncertain within the quantum fluctuation. However, in the event that pseudo-random number which is used for regular cryptography is used in a process of varying the basis, it has been reported that the security is merely the level of regular classical cryptography when the number of photons per signal is large (Non-Patent Document 3: T. Nishioka, T. Hasegawa, H. Ishizuka, K. Imafuku, and H. Imai, Physics Letters A 327, 28-32 (2004)). Accordingly, the system is still in the research phase.
The method by Yuen et al. breaks away from utilization of the faint light with one or less photon and is invented by bearing in mind that not only keys are distributed but also signals themselves are transmitted, and it can be said that it is the invention which comes close to the realistic standpoint. However, this method is not premised on the macroscopic light intensity as used in general optical communication systems. A still more advanced invention is required to introduce a quantum-mechanical system into the general optical communication systems.
Because the quantum-mechanical properties generally become conspicuous in microscopic areas, when the light intensity is made macroscopic, the quantum-mechanical properties are generally difficult to be exhibited. As an optical state which indicates quantum-mechanical properties even at the macroscopic light intensity, a squeezed state is known. The squeezed state is a state that the fluctuation of a vacuum or a coherent state is controlled. Laser output light is well described by the coherent state (The fluctuation of a coherent state is equal to that of a vacuum). In a vacuum (a coherent state), the size of two quadrature fluctuation components is equal, whereas under the squeezed condition, one of the quadrature fluctuation components is smaller than a vacuum fluctuation and the other fluctuation component is larger than that. An area of vacuum fluctuation (fluctuation of the coherent state) in a quadrature phase space is the minimum that cannot be made smaller any more, and a noise level based on the vacuum fluctuation (also called quantum fluctuation) is called a standard quantum limit. The component of the reduced fluctuation in the squeezed state has broken the standard quantum limit and has been attracting researchers' attention. However, for example, when the squeezed state is under a loss process, the quadrature phase component whose fluctuation is small becomes easily the level of a vacuum fluctuation (fluctuation of a coherent state) due to an inflow of a vacuum fluctuation. Consequently, it is nearly impossible to apply the squeezed state to optical communications in which loss is unable to be avoided if the reduced fluctuation component is focused our attention on. On the other hand, with respect to the large fluctuation component in the squeezed state (antisqueezed component), the general properties of the fluctuation are determined by the originally expanded fluctuation even when a vacuum fluctuation is added due to loss. Even if there is any loss, although the fluctuation is reduced to the extent of the loss, it does not easily regress to the level of vacuum fluctuation (fluctuation of the coherent state). That is, the antisqueezed component has loss-tolerance nearly equal to the general classical optical communications. By similar discussion, also for an optical amplification, the component of the fluctuation reduced to a level equal to or less than that of the vacuum fluctuation is unable to maintain its properties, whereas the component of expanded fluctuation have loss-tolerance. The optical communication method which makes it difficult to be eavesdropped on by the use of the expanded fluctuation component is set forth in unpublished Patent Document 1 (Japanese Patent Laid-Open Publication No. 2006-191410).
According to the method of Patent Document 1, the signals are binary and the axes corresponding to the bases are randomly chosen in a phase space. The negative and positive directions of each base axis correspond to binary signals, and fluctuation is expanded in a direction perpendicular to the base axes. The random properties of the base axes are assumed to be able to be known by a legitimate receiver and, by using information on the bases, the legitimate receiver projects the signals in a direction of being not subjected to the expanded fluctuation and measures the projected signals (regular homodyne detection). On the premise that the legitimate receiver knows the random base axes, it never becomes difficult to receive the signals and because the signals are superimposed in the direction perpendicular to the expanded fluctuation, the S/N ratio of signals is never degraded. On the other hand, even if there is any eavesdropper, unless the eavesdropper has the information concerning the randomized bases, the eavesdropper detects signals including the expanded fluctuation and accordingly the S/N ratio is dramatically degraded. That is, because a bit error rate of the eavesdropper is significantly increased as compared to that of the legitimate receiver, security of communications is reinforced to such a degree.
In order that the sender and the receiver share information for the signal bases, a pseudo-random number generator using a seed key is used. The pseudo-random number generation using the seed key is the method generally adopted in the present cipher communications. The method using the antisqueezed light is a communication method with security reinforced by using the physical laws, wherein additional difficulty of eavesdropping is added to the security of regular cipher communications, based on the expanded fluctuation. Furthermore, in this method, if the antisqueezing strength is increased in accordance with the signal intensity, the similar bit error rate can be achieved, irrespective of the light intensity. Therefore the above method is the communication method which has quantum mechanical properties applicable even to the macroscopic number of photons as used in the regular optical communications.
Because the fluctuation of one quadrature phase component can be automatically expanded when squeezed light is generated, it is basically preferable to generate the squeezed light for generating antisqueezed light. Squeezed light-generating methods which have been invented thus far are primarily intended for squeezing, but are not intended to generate the antisqueezed light with macroscopic amplitude (intensity) for carrying out the secure optical communications. For example, a method using degenerate parametric down conversion has been a method in which 2ω-angular-frequency light is used as pump light to amplify ω-angular-frequency light of no input signal (vacuum). The process depends on the phase and a vacuum is squeezed by approximately several dB (Non-Patent Document 4: L. Wu, M. Xiao, and H. J. Kimble, Journal of Optical Society of America 4, 1465-1475 (1987) and Non-Patent Document 5: T. Hirano and M. Matsuoka, Optics Letters 15, 1153-1155 (1990)). Various methods using Kerr effects of an optical fiber have been proposed. The Kerr effect is a phenomenon in which refractive index varies in accordance with light intensity. Refractive index is given by n=n0+n2I, where I denotes light intensity. Herein, n0 denotes linear refractive index and n2 is the coefficient that gives the nonlinear refractive index. In the event that there is any fluctuation in a light amplitude direction (intensity), the refractive index is fluctuated by the Kerr effect and, as a result, the fluctuation is expanded in the phase direction. In the event that the expanded fluctuation in the phase direction is sufficiently small as compared to its amplitude, the fluctuation will be varied so that an area of fluctuation in the quadrature phase space is maintained. At this time, one direction of the fluctuation is reduced due to the expansion of the fluctuation in the phase direction. That is, the light is squeezed. A method intended to generate quasi-squeezed vacuum by using the principles as described above is a method of a symmetric type fiber interferometer (Non-Patent Document 6: M. Shirasaki and H. A. Haus, J. Opt. Soc. Am. B7, 30-34 (1990) and Non-Patent Document 7: C. X. Yu, H. A. Haus, and E. P. Ippen, Optics Letters 26, 669-671 (2001)), whereas a method intended to generate the light squeezed in the amplitude direction, having amplitude, is a nonsymmetrical type fiber interferometer (Non-Patent Document 8: M. J. Werner, Physical Review Letters 19, 4132-4135 (1998) and Non-Patent Document 9: S. Schmitt, J. Ficker, M. Wolff, F. Koenig, A. Sizmann, and G. Leuchs, Physical Review Letters 81, 2446-2449 (1998)).
These methods are primarily intended to generate the squeezed light and are not been intended to generate the antisqueezed light for the optical communications. Some methods may require solid-state laser or fiber laser as pump light or require even a high-stability optical interferometer. In addition, they do not always satisfy the requirements of long-term reliability, maintenance-free operation, a high-repetition rate, low jitter, and inter-pulse coherence, etc. which are required in the optical communications. Consequently, to achieve the secure optical communications as disclosed in Patent Document 1, any invention on the antisqueezed light source is essential.